In 1895 the Russian painter Nikolai Bogdanov-Belsky exhibited a painting called Mental Arithmetic. In the Public School of S. A. Rachinsky. In a village classroom, barefoot peasant boys stare at the blackboard, some with a hand on their chin, one whispering in the teacher’s ear. On the blackboard is written: $$\frac{10^2 + 11^2 + 12^2 + 13^2 + 14^2}{365}.$$ Nikolai Bogdanov-Belsky, Mental Arithmetic. In the Public School of S. A. Rachinsky (1895). Tretyakov Gallery, Moscow.Nikolai Bogdanov-Belsky · Wikimedia Commons · public domain The teacher really existed. Sergei Rachinsky was a professor of botany at Moscow University who gave up his career to teach peasant children in the village of Tatevo, where he had been born. He liked to set sums that looked enormous and had a shortcut. Bogdanov-Belsky himself had been one of his pupils.